The field of the invention relates generally to friction stir welding and more particularly to friction stir welding of dissimilar metals and to workpiece assemblies formed by friction stir welding dissimilar metals together.
Recent surveys conducted by the Joining and Welding Research Institute (JWRT) of Japan and the Edison Welding Institute (EWI) of the U.S. have identified welding of dissimilar metals as a top priority in materials joining technologies. For instance, being able to weld aluminum-to-copper would be advantageous in many industries where electric connections are made during the manufacturing process. In another instance, being able to weld aluminum-to-steel or aluminum-to-magnesium would result in significant weight reduction in some applications, which would be advantageous in many industries, e.g., the aircraft, locomotive, shipbuilding, and automotive manufacturing industries. Being able to efficiently and effectively weld aluminum-to-magnesium is of particular interest because magnesium has a specific strength (i.e., strength to density ratio) that is 14 percent higher than aluminum and 68 percent higher than iron making it one of the lightest metallic structural materials. Although aluminum and magnesium alloys are typically soft materials and often have relatively similar melting points, they tend to react with each other when heated such as during friction stir welding. The more they are heated up, that is, the more heat input during friction stir welding, the more they react with each other to weaken the resultant weld.
As illustrated in FIGS. 1 and 2, friction stir welding is typically preformed using a cylindrical tool T having a pin P extending downward therefrom. During use, the pin P is rotated at a constant speed and fed at a constant traverse rate along a joint line JL between two pieces of sheet or plate material P1, P2. The pieces P1, P2 can be butt welded (side-to-side) together, as illustrated in FIG. 1, or lap welded (overlapped) together, as illustrated in FIG. 2. The pin P of the tool T typically has a length that is slightly less than the desired weld depth. During the welding process, a shoulder S of the tool T is often in direct contact with an upper surface of both the pieces P1, P2 being welded. As a result, heat is generated by the friction between the tool shoulder S and pin P and the pieces P1, P2 being welded. This heat causes the pieces P1, P2 to soften about the joint line JL without reaching their melting point. In other words, the friction stir welding process causes both of the pieces P1, P2 to plasticizes adjacent the joint line JL. As the tool T is fed transversely with respect to the pieces P1, P2, a leading face of the pin P directs the plasticized material toward a back of the pin. Angling the pin P by 2-4 degrees in the transverse direction facilitates feeding the pin through the pieces P1, P2 and directing the plasticized material toward the back of the pin.
With reference to FIG. 1, prior efforts have been made to butt weld an aluminum alloy to a magnesium alloy using friction stir welding. In conventional butt welding using friction stir welding, an end of a piece of aluminum alloy (e.g., piece P1) is accurately aligned with and placed against an end of a piece of magnesium alloy (e.g., piece P2) to form a seam or joint line TL. The tool T and thereby the pin P are fed along the joint line JL (i.e., in a longitudinal direction with respect to the two pieces P1, P2 as viewed in FIG. 1). The pin P can be coaxial with the joint line JL or offset toward either the piece of aluminum alloy P1 or the piece of magnesium alloy P2. In addition, the piece of aluminum alloy P1 can be placed on either the advancing side, as illustrated in FIG. 1, or retreating side of the tool T. The piece of magnesium alloy P2 is placed on the opposite side from the piece of aluminum alloy P1. In the illustrated configuration, the piece of magnesium alloy P2 is placed on the retreating side of the tool T. While the butt weld formed between the pieces of aluminum and magnesium alloys P1, P2 can be relatively strong, strict tolerances and controls are needed during the welding process to form such a weld thereby making it difficult, relatively time consuming, and costly.
As illustrated in FIG. 2, prior efforts have also been made to lap weld a piece of aluminum alloy (e.g., piece P1) to a piece of magnesium alloy (e.g., piece P2). Lap welding is typically preferred by manufacturers because the tolerances and controls needed during the welding process are substantially less than those needed during the butt welding process. As seen in FIG. 2, lap welding is preformed by overlapping a portion of one of the pieces with a portion of the other piece. In the illustrated configuration, the piece of magnesium alloy P2 overlaps the piece of aluminum alloy P1. The overlapped portions of the pieces P1, P2 are welded together using friction stir welding. That is, the overlying piece P2 is friction stir welded to the underlying piece P1.
The pieces P1, P2 can be welded together along a single joint line using conventional single-pass lap welding or can be welded together along two joint lines using conventional double-pass lap welding. When conventional double-pass lap welding is used, the pieces P1, P2 of materials are flipped over after they have been welded on one side and are then welded on the opposite side using the same process. When dissimilar metals (e.g., aluminum alloy and magnesium alloy) are lap welded together using friction stir welding, brittle intermetallic compounds (e.g., Al12Mg17, Al3Mg2) are often formed, which severely degrades the strength of the weld. As a result, prior efforts to lap pieces of dissimilar materials have been relatively unsuccessful.